Physics Essay 1.8 Fall, 2009

The Eye

What are the parts of the eye? How does the spacial arrangement of these parts effect your vision and color perception?

The diagram of the eye below shows the important parts of the eye. The part of the eye that registers light from the outside world is called the retina and is shown in blue. The retina covers the entire inside-back portion of the eye. Rod cells and cone cells are embedded in, and distributed across the retina and these cells are the parts that actually register light. When light hits these cells, the cells send a signal to the brain that light is striking a certain part of the retina. In this way the retina acts a bit like a screen and when the brain receives electrical signals from the rod and cone cells the brain knows what kind of light is hitting different parts of the retina.

It is important that the rod cells cannot distinguish between different kinds of colors. These rod cells can only register the intensity or brightness of the light that strikes them. It is the cone cells that can register color. There are three different kinds of cone cells, those that register red light, those that register green light, and those that register blue light. Thus when bright light strikes a rod cell it sends an electrical signal to the brain and the brain knows that bright light is hitting that portion of the retina. If this bright light is blue then the cone cell right next door to a rod cell sends a signal to the brain that the light striking this potion of the retina is blue. With the two pieces of information the brain knows that bright blue light is striking that portion of the retain.

Figure 1. The eye, showing the most important parts involved in forming images and seeing color.

It is the optic nerve that carries the the electrical signals to the brain and the iris can change size to let more or less light into the eye, through the hole in the middle of the iris. The hole in the middle of the iris is called the pupil. The lens of the eye can refract incoming light to help form clear images. We will experiment with lenses after we experiment with optical illusions and mirrors. The eye is filled with a liquid like Vitreous Gel.

The rod and cone cells can be referred to as photochemical receptors. The three different types of cone cells (red receptors, blue receptors, and green receptors) can work in concert to help perceive colors such as yellow, magenta, and cyan. When red and blue light strikes a certain portion of the retina the red-reading cone cells starting working and send signals to the brain. The blue-reading cone cells in the this region do the same, but the green-reading cone cells in this region remain “off” and send no signal to the brain. Thus the brain ‘sees’ magenta in this region.

The nerve pathways that carry the signals to the brain are made-up of individual nerve cells. When triggered, each nerve cell releases a chemical that travels across a gap to the next nerve cell. This chemical then triggers the next nerve cell in the pathway, allowing the signal to move along the pathway to the brain. Other chemicals are then released to ‘neutralize’ the initial chemical and end the signal, but this can take time to accomplish. Cones that have been subjected to a certain kind of light can become overworked, and over time send weaker signals to the brain.

Your job in the Optical Illusion Lab is to combine this new knowledge about the eye with what you know about reflection, refraction and light/color mixing and explain how the different illusions are created.